The official Pokémon website has officially confirmed and revealed the new Eevee evolution as Sylveon.

Sylveon
Type to be Revealed
English: Sylveon Spanish: Sylveon Italian: Sylveon French: Nymphali German: Feelinara Japanese: Nymphia (ニンフィア)

A New Pokémon Has Been Revealed!

Meet Sylveon, a newly discovered evolution of Eevee! Eevee first appeared in the original Pokémon video games, Pokémon Red and Pokémon Blue. Experienced Pokémon Trainers know that Eevee can evolve into seven different types, and now a new Eevee evolution with a different type has been revealed! What type is Sylveon? You'll have to wait to find out!

Trailer

Source: http://www.youtube.com/watch?v=pse_I1klRhA